Plastic bags don't weigh very much. It would take thousands or tens of thousands of bags to make a single gallon of fuel. Based on the annual consumption of plastic bags in the US and a 100% efficient production process, we could make 6 million gallons of fuel from old plastic bags. That's fuel to power one average size county in rural Iowa for one year. Looks like we need to buy more groceries to fuel the rest of the country.